Check if user is already logged in asp.net website

Question

I am developing a website where in user is required to login to the system to be able to use it. The current functionality is: When user enters username and password, a check is

Answer 1

This is how I managed to do it with the help of link provided by @Satinder singh.

Global.asax.cs

    protected void Application_Start(object sender, EventArgs e)
    {
        Application["UsersLoggedIn"] = new System.Collections.Generic.List();
    }

    protected void Session_End(object sender, EventArgs e)
    {
        // NOTE: you might want to call this from the .Logout() method - aswell -, to speed things up
        string userLoggedIn = Session["UserLoggedIn"] == null ? string.Empty : (string)Session["UserLoggedIn"];
        if (userLoggedIn.Length > 0)
        {
            System.Collections.Generic.List d = Application["UsersLoggedIn"]
                as System.Collections.Generic.List;
            if (d != null)
            {
                lock (d)
                {
                    d.Remove(userLoggedIn);
                }
            }
        }
    }

Login.aspx.cs

protected bool Login(string userId)
    {
        System.Collections.Generic.List d = Application["UsersLoggedIn"]
            as System.Collections.Generic.List;
        if (d != null)
        {
            lock (d)
            {
                if (d.Contains(userId))
                {
                    // User is already logged in!!!
                    string userLoggedIn = Session["UserLoggedIn"] == null ? string.Empty : (string)Session["UserLoggedIn"];
                    if (userLoggedIn == user_id)
                    {
                        Session["UserLoggedIn"] = user_id;
                        return true;
                    }
                    else
                    {
                        return false;
                    }
                }
                else
                {
                    string userLoggedIn = Session["UserLoggedIn"] == null ? string.Empty : (string)Session["UserLoggedIn"];

                    if (userLoggedIn != user_id)
                    {
                        d.Add(userId);
                    }
                }
            }
        }
        Session["UserLoggedIn"] = userId;
        return true;
    }

With above code, I am allowing any user to be logged in at any time only once. I used session variable to check if the request is from same browser, if so, I am allowing him/her to login else throwing a message "You are already logged in from another system".